A digital communication system typically involves transmitting a modulated data stream from a transmitter to a receiver over a communication channel. The communication channel can include a microwave radio link, a satellite channel, a fiber optic cable, or a copper cable to provide some examples. A communication channel contains a propagation medium that the modulated data stream passes through before reception by the receiver.
The propagation medium of the communication channel introduces distortion into the transmitted modulated data stream causing a received modulated data stream to differ from the transmitted modulated data stream. Noise, signal strength variations known as fading, phase shift variations, or multiple path delays known as multi-path propagation can introduce distortion into the transmitted modulated data stream. For example, transmission over a multiplicity of paths of different and variable lengths, or rapidly varying delays in the propagation medium from the transmitter to the receiver, may cause a change in the amplitude and/or phase of the transmitted modulated data stream. The distortion caused by the communication channel may be characterized as either static distortion or dynamic distortion. Static distortion occurs when the communication channel does not substantially fluctuate with time. Dynamic distortion occurs when the communication channel fluctuates over time and may be characterized as fast or slow depending on the rate of fluctuation. Different types of distortion tend to fluctuate at different rates. For example, the distortion due to multi-path propagation might be characterized as slow dynamic distortion, whereas the distortion due to phase shift variations might be characterized as fast dynamic distortion.
Digital communication systems use an adjustable filter in the form of an equalizer to reduce the effect of the distortion caused by the communication channel. A receiver may directly set equalization filter coefficients for known or measured communication channels. However, in most situations the characteristics of the communication channel are not known in advance and therefore require the use of an adaptive equalizer. Adaptive equalizers derive adjustable filter coefficients from a received demodulated data stream. The adaptive equalizer may compensate for the distortion caused by the communication channel provided that the distortion is either a static distortion or a slowly fluctuating dynamic distortion. However, the adaptive equalizer is not well suited to compensate for a more rapidly fluctuating distortion, such as distortion due to phase variations.
Conventional equalizer outputs feed their corresponding outputs to a coefficient update module to adjust the equalizer coefficients. If the equalizer output contains fast dynamic distortion components such as phase variations that cannot be compensated by the equalizer, the equalizer coefficients may not be properly updated, and therefore the equalizer may not be able to compensate even slowly fluctuating distortion in the received signal.
To properly update its coefficients in the presence of either fast or slow dynamic phase variations, the adaptive equalizer may operate in conjunction with a phase correction circuit. The imaginary part of one of the equalizer coefficients is constrained to prevent the adaptive equalizer from attempting to correct for the phase variations. The phase correction circuit uses the equalizer output to correct for phase variations then a phase corrected data stream is used to update the equalizer filter coefficients.
Current digital communication systems may operate in lower signal-to-noise ratio conditions. For example, advances in error correction coding allow error free operation for digital communication systems at lower signal-to-noise ratios. This presents a challenge for the conventional phase correction circuits used in current receiver architectures. Conventional phase correction circuits are typically decision based phase locked loops that may not correct the phase of the received demodulated data stream under these lower signal-to-noise ratio conditions.
Therefore, what is needed an adaptive equalizer that is capable of compensating for fast or slow dynamic phase distortion in lower signal-to-noise ratio environments.
The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.